1. Technical Field of the Invention
The present invention relates to a packed and frozen sushi product being comprised of a shaped form of frozen boiled rice, a frozen sushi material which is preferably not heated, onto the frozen shaped form of boiled rice and a plastic films packing said frozen sushi material and frozen boiled rice, and a process for thawing the same. The packed and frozen sushi product according to the present invention is made by placing the sushi material (Gu) onto the shaped form of boiled rice, packing said sushi material and boiled rice with plastic film and freezing said sushi material and boiled rice together with plastic film. Further, packed and frozen sushi product according to the invention is made by placing the sushi material on the shaped boiled rice, freezing said sushi material boiled rice and packing said frozen sushi material and boiled rice with plastic film. More particularly, the present invention relates to a process for thawing a packed and frozen sushi product, which is capable of heating the shaped form of frozen boiled rice by an electromagnetic wave heating in a microwave over or the like to thaw the sushi material by a remaining heat of the shaped form of boiled rice. Further, the present invention relates to a packed and frozen sushi product which is suitable for thawing, and a process for thawing the same.
Yet further, the present invention relates to a process for thawing a packed and frozen sushi product and more particularly, to a process for thawing a packed and frozen sushi product, which includes heating a packed and frozen sushi product by an electromagnetic wave heating in a microwave oven and the like to thaw a sushi material by a remaining heat of the shaped form of boiled rice.
2. Description of the Related Art
In general, for example, to make a rod-like sushi product, a vinegared boiled rice is first shaped. Then, a sushi material comprised of a prepared boned fish such as a porgy, a salmon, a mackerel and the like is placed on the shaped form of rice such as vinegared boiled rice along with a white thin sheet of kelp (kombu in Japanese) made by boiling a piece of kelp in a stock seasoned by vinegar, sugar and salt, and the resulting mass is shaped. This shaped sushi mass is inserted into and packed in a bag made of a plastic film in a degassed manner. The resulting mass is frozen by brine, thereby producing a frozen sushi product comprising the frozen sushi material and the frozen boiled rice portion. Alternatively, a sushi shaped into a rod-like configuration is frozen by a liquid nitrogen or a liquefied carbon dioxide gas to freeze the sushi material and the boiled rice portion. This frozen rod-like sushi is placed into and packed in a bag made of a plastic film in a degassed manner, thereby producing a frozen sushi product with the sushi material and rice portion frozen.
For example, in such a sushi product frozen by brine, the degassing packing is carried out prior to the freezing and hence, the deposition of bacteria, the absorption of odor and the like do not occur in a freezing chamber. Therefore, this frozen sushi product is excellent in a sanitary aspect, as compared with a frozen sushi product made through a slow freezing by air blast or by liquid nitrogen or liquefied carbon dioxide gas, followed by a degassing packing. Thus, the sushi product frozen by brine can be stably preserved over a long period of three months or more at a preservation temperature of -18.degree. C. and further, can be preserved over a further long period of six months at a preservation temperature of -50.degree. C.
However, the frozen sushi product preserved as described above is thawed for use. Examples of the thawing method are a natural thawing method, a heating thawing method using a flowing ice, a heating thawing method using steam, a heating thawing method using warm water, an electromagnetic wave heating thawing method in a microwave oven, and the like. In the electromagnetic wave heating thawing method in the microwave oven, the thawing time is extremely short and the thawing yield is high. Therefore, the electromagnetic wave heating thawing method in the microwave oven is suitable for thawing the frozen sushi product. For example, for the rod-like sushi product, the natural thawing thereof at room temperature requires five hours, but the electromagnetic wave heating thawing method in the microwave oven can thaw the rod-like sushi product within a short time, e.g., four minutes, depending upon the type of the sushi material. Therefore, the thawing in the microwave oven is convenient, because it can immediately meet the demand for the sushi in an eating house.
When the thawing in the microwave oven is used, different heat values are generated at the frozen sushi material portion and the frozen boiled rice portion by absorption of an electromagnetic wave such as a high frequency wave. Moreover, a surface portion of the sushi product is heated relative to a central portion and hence, a portion of the sushi material is heated up to an undesirable temperature. Therefore, the thawing in the microwave oven has not been practically used.
Furthermore, in the case of a hand-shaped sushi, it is preferred that a boiled rice portion has a body warmth (30.degree. to 350.degree. C.), while a sushi material portion has a temperature of 10.degree. to 13.degree. C., but in the case of a rod-like sushi, it is preferred that a boiled rice portion such as vinegared boiled rice and a sushi material portion have a temperature of 20.degree. to 25.degree. C. When the electromagnetic wave thawing method in the microwave over or the like is utilized for thawing a packed and frozen sushi product such as a hand-shaped sushi product or a rod-like sushi product, the sushi material of the sushi product is heated to a relatively high temperature, resulting in an injured taste of the sushi product. Therefore, it is relatively difficult to achieve the intended thawing.